Somewhat similar are the northers, or blizzards, of our Western States. By the laws already given, it is seen that northerly winds can prevail in any region only when some region further south is unusually heated. Now, the northern portion of America may be roughly compared to a trough. The cold polar current sets to the southward across the continent, and is turned to the east by the Rocky Mountain range, giving it a general southeast course. Hence, when the southern summer is in progress, our prevailing winds are from the northwest; and when the heated portion of the world is north of the equator, we have the return trade, giving us as our prevailing wind that from the southwest. When our return trade is unusually prolonged, we have a late fall; and if the southern summer is unusually warm, we have the polar current longer than usual, and a late spring in consequence. The polar current seldom makes its presence felt beyond the Texan plains; though occasionally it reaches the Mexican plateau, or sweeps across the Gulf to the Antilles. A similar cold wind from Central France toward the Riviera is locally known as the Mistral. The cold winds from the south, which in crossing the plains of Patagonia, are turned eastward by the Andes, are called in Uruguay the _pamperos_, as their direction causes the popular belief that they originate in the pampas, or grassy plains. In Malta the cold wind becomes known as the _gregale_--in the Adriatic sea it is the _tramontana_; in Trieste and Dalmatia it is the _bora_. In New Zealand the corresponding cold blast comes from the south, and is known as the buster. When loaded with drifting snow, as in the blizzard of the United States, the cold wind of the Yenisei Valley, in Asia, is locally called the _purga_; in the steppes of Central Asia it is the _bura_. Eastern Asia receives its prevailing cold current from the northwest; while western Asia and Europe receive their cold wave from the northeast, there being no range of mountains, as in America, to deflect the current, as the polar currents are disposed to follow the continents, having their origin in arctic lands; while for a similar reason the return trades reach their extremes on the ocean. Hence, lines drawn through the places which possess the same mean annual temperature reach a higher latitude at sea than on land. These are the chief periodical winds of long periods. There is one other class to be noted: the diurnal land and sea breezes. These occur along all coasts, whether in the zone of trades or of variable winds; but the phenomenon is more strongly marked in the tropical regions, and in the summer of the temperate latitudes, because of the greater difference in the temperature of land and sea by day and by night.